1. Field of the Invention
The present invention relates to a control apparatus for a vehicle equipped with an electronic throttle control unit (also called a drive by wire) and an automatic transmission, and more particularly to a vehicle control apparatus adapted to be employed when the electronic throttle control unit has failed.
2. Description of Related Art
In engines such as automobile engines, a drive by wire (hereinafter referred to as a DBW) connecting an accel pedal and a throttle valve together by an electrical signal has hitherto been developed. In a DBW such as this, the accel pedal and the throttle valve are not mechanically connected, and based on a variety of parameters in addition to an operational quantity of the accel pedal (accel opening angle), a virtual accel opening angle ("pseudo" accel opening angle) is set by a computer. In correspondence with this, the throttle valve can be controlled, and the DBW is also referred to as an electronic throttle control unit.
Therefore, for example, during idling where the accel pedal has not been operated (i.e., the accel opening angle is less than a small predetermined value), idling engine speed can be controlled, while the throttle valve is being adjusted finely. Also, in accordance with the traveling state of the vehicle and the operating state of the engine, the accel opening angle (driver's operation) is corrected in order to set a pseudo accel opening angle. With the control of the throttle valve based on this, engine running with a feeling of smoothness is realizable.
On the other hand, a spark ignition type in-cylinder injection internal combustion engine (hereinafter referred to as an engine), which is an internal combustion engine igniting sparks by spark plugs (generally, a gasoline engine) and injects fuel directly into cylinders, has been put to practical use in recent years. In such an engine, an enhancement in the engine fuel consumption performance and an enhancement in the output performance are compatible with each other by making use of the characteristic that fuel injection timing can be freely performed and also the formed state of an air-fuel mixture can be freely controlled.
In other words, in this spark ignition type in-cylinder injection engine, fuel is injected on the compression stroke, and with this, an operation in a state in which fuel is extremely lean (i.e., a super-lean combustion operation in which an air-fuel ratio is extremely higher than a stoichiometric air-fuel ratio) can be performed by stratified-charge combustion. The engine is provided with a super lean operating mode (compression stroke injection mode or lean compression operating mode) as the combustion form and can realize a considerable enhancement in the fuel consumption ratio.
Of course, the spark ignition type in-cylinder injection engine can also perform a premixed combustion operation in which fuel is injected primarily on the intake stroke. In this case, fuel is injected directly into the combustion chamber (cylinder), whereby the greater part of fuel injected at each combustion cycle can be burned with reliability within the combustion cycle. The engine, therefore, can also enhance the engine output.
A premixed combustion operation such as this can also set as combustion form a lean operating mode (lean intake operating mode) which performs operation in a fuel-leaned state which is not so lean as the super lean operating mode (i.e., in a state in which an air-fuel ratio is higher than a stoichiometric air-fuel ratio), a stoichiometric operating mode (stoichiometric feedback operating mode) which performs feedback control on the basis of O.sub.2 -sensor information so that an air-fuel ratio reaches a stoichiometric air-fuel ratio, and an enriched operating mode (open-loop operating mode) which performs operation in a fuel-enriched state (i.e., in a state in which an air-fuel ratio is lower than a stoichiometric air-fuel ratio).
In general, if a requested output to the engine is small, i.e., if the revolution speed of the engine is low and also the engine load is low, the lean compression operating mode will be selected in order to enhance fuel consumption. As the engine revolution speed and the engine load increase further, the lean intake operating mode, the stoichiometric operating mode, and the enriched operating mode are selected in the recited order.
In the case of the supper lean combustion operating mode (lean compression operating mode), incidentally, there is a need to supply more air to the combustion chamber in order to make an air-fuel ratio high. However, in this lean compression operating mode, since operation is performed in an area where engine load is low, i.e., in an area where the stepping-on quantity of the accel pedal (accel opening angle) is small, the opening angle of the throttle valve corresponding to the accel opening angle cannot satisfy a required air-fuel ratio.
Hence, a technique where an electronically controlled valve (air bypass valve) is arranged in an air bypass passage bypassing an intake-air passage equipped with the throttle valve has been developed. In this technique, when intake air is insufficient at the opening angle of the throttle valve corresponding to the accel opening angle, the air bypass valve is opened in correspondence with a required air volume, thereby performing air supply.
Incidentally, applying the aforementioned DBW to the above-mentioned spark ignition type in-cylinder injection engine is also considered. In other words, in the DBW, since the opening angle of the throttle valve can be controlled without corresponding to the accel opening angle, more air than a quantity corresponding to the accel opening angle can be supplied to the combustion chamber. Therefore, for example, in the lean compression operating mode of the spark ignition type in-cylinder injection engine, even if the accel opening angle is small, a necessary quantity of air can be supplied to the combustion chamber.
However, in the case where such a DBW is adopted, it is desirable to prepare a counter plan against an unlikely failure of the DBW as well.
As failure causes of this DBW, it is considered that the throttle valve which is controlled by the DBW is stuck by catching foreign material, such as dust, contained in blow-by gas or exhaust gas recirculated by an exhaust gas recirculation (EGR) system, etc.
For example, if the throttle valve is stuck in a nearly fully opened state, the opening angle of the throttle valve cannot be controlled by the DBW. Consequently, even in the case where the driver does not request engine output, i.e., even in the case where the stepping-on quantity of the accel pedal is small, the engine output torque is not reduced. For this reason, in this case, if a low speed gear stage is selected, the vehicle driving torque will not be reduced and therefore engine output against the driver's intention will be produced, resulting in a problem that stable travel cannot be ensured.
In such a case, if a high speed gear stage is selected, the vehicle driving torque will be considerably reduced and therefore the engine output will be insufficient when the vehicle is started. Likewise, there is a problem that stable travel cannot be ensured.
On the other hand, if the throttle valve is stuck in a nearly fully closed state, the opening angle of the throttle valve cannot be controlled by the DBW. Consequently, even in the case where the driver is requesting engine output, i.e., even in the case where the stepping-on quantity of the accel pedal is great, the engine output torque is not raised. For this reason, in this case, if a high speed gear stage is selected, the vehicle driving torque will be considerably reduced and therefore there will be a problem that stable travel corresponding to the driver's request cannot be ensured.
In Japanese Laid-Open Patent Publication No. HEI 2-195070, there is disclosed a technique which controls the speed change stages of an automatic transmission in the case where the operating state of a throttle valve is judged to be abnormal.
However, this technique is one which adjusts the speed change stage of the automatic transmission to a predetermined speed change stage corresponding to either vehicle speed or engine revolution speed in the case where the operating state of a throttle valve is judged to be abnormal, and no consideration has been given to the failure state of the throttle valve.
Japanese Laid-Open Patent Publication No. HEI 6-249332 discloses a technique which controls the position of the speed change gear stage of the automatic transmission in consideration of the failure state of the throttle valve. In this technique, in order to obtain sufficient driving force when the throttle control unit is in a state which cannot open the throttle valve, the highest stage position of an allowed speed change gear stage is decided based on the acceleration intention of the driver, and from among speed change gear stage positions equal to or less than this highest stage position of the speed change gear, a speed change gear stage position is selected so that the maximum driving force is obtained. However, this technique has taken neither an enhancement in safety nor an ensurance of stable travel into consideration.